Environmental DNA (eDNA) studies have burgeoned over the last two decades and the application of eDNA has increased exponentially since 2010, albeit at a slower pace in the marine system. We provide a literature overview on marine metazoan eDNA studies and assess recent achievements in answering questions related to species distributions, biodiversity and biomass. We investigate which are the better studied taxonomic groups, geographic regions and the genetic markers used. We evaluate the use of eDNA for addressing ecological and environmental issues through food web, ecotoxicological, surveillance and management studies. Based on this state of the art, we highlight exciting prospects of eDNA for marine time series, population genetic studies, the use of natural sampler DNA, and eDNA data for building trophic networks and ecosystem models. We discuss the current limitations, in terms of marker choice and incompleteness of reference databases. We also present recent advances using experiments and modeling to better understand persistence, decay and dispersal of eDNA in coastal and oceanic systems. Finally, we explore promising avenues for marine eDNA research, including autonomous or passive eDNA sampling, as well as the combined applications of eDNA with different surveillance methods and further molecular advances. Keywords: environmental DNA, DNA metabarcoding, marine metazoa, biodiversity, population genetics, natural sampler DNA, diet analysis.

Global warming causes profound environmental shifts in the Arctic Ocean, altering the composition and structure of communities. In the Fram Strait, a transitional zone between the North-Atlantic and Arctic Ocean, climate change effects are particularly pronounced and accelerated due to an increased inflow of warm Atlantic water. Gelatinous zooplankton are known as key predators, consuming a great variety of prey and playing an important role in marine ecosystems. Insufficient knowledge of how gelatinous zooplankton are affected by environmental change has resulted in a notable gap in the understanding of the future state of Arctic ecosystems. We analyzed the diversity and abundance of gelatinous zooplankton down to 2600 m depth and established the first regional baseline dataset using optical observations obtained by the towed underwater camera system PELAGIOS (Pelagic In situ Observation System). Our data estimate the abundance of 20 taxa of gelatinous zooplankton. The most abundant taxa belong to the family of Rhopalonematidae, mainly consisting of Aglantha digitale and Sminthea arctica, and the suborder Physonectae. Using the observational data, we employed a joint species distribution modelling approach to better understand their distributional patterns. Variance partitioning over the explanatory variables showed that depth and temperature explained a substantial amount of variation for most of the taxa, suggesting that these parameters drive diversity and distribution. Spatial distribution modelling revealed that the highest abundance and diversity of jellyfish are expected in the marginal sea-ice zones. By coupling the model with climate scenarios of environmental changes, we were able to project potential changes in the spatial distribution and composition of gelatinous communities from 2020 to 2050 (during the summer season). The near-future projections confirmed that with further temperature increases, gelatinous zooplankton communities in the Fram Strait would become less diverse but more abundant. Among taxa of the Rhopalonematidae family, the abundance of Aglantha digitale in the entire water column would increase by 2%, while a loss of up to 60% is to be expected for Sminthea arctica by 2050. The combination of in situ observations and species distribution modelling shows promise as a tool for predicting gelatinous zooplankton community shifts in a changing ocean.

In times of accelerating climate change, species are challenged to respond to rapidly shifting environmental settings. Yet, faunal distribution and composition are still scarcely known for remote and little explored seas, where observations are limited in number and mostly refer to local scales. Here, we present the first comprehensive study on Eurasian-Arctic macrobenthos that aims to unravel the relative influence of distinct spatial scales and environmental factors in determining their large-scale distribution and composition patterns. To consider the spatial structure of benthic distribution patterns in response to environmental forcing, we applied Moran’s eigenvector mapping (MEM) on a large dataset of 341 samples from the Barents, Kara and Laptev Seas taken between 1991 and 2014, with a total of 403 macrobenthic taxa (species or genera) that were present in ≥ 10 samples. MEM analysis revealed three spatial scales describing patterns within or beyond single seas (broad: ≥ 400 km, meso: 100–400 km, and small: ≤ 100 km). Each scale is associated with a characteristic benthic fauna and environmental drivers (broad: apparent oxygen utilization and phosphate, meso: distance-to-shoreline and temperature, small: organic carbon flux and distance-to-shoreline). Our results suggest that different environmental factors determine the variation of Eurasian-Arctic benthic community composition within the spatial scales considered and highlight the importance of considering the diverse spatial structure of species communities in marine ecosystems. This multiple-scale approach facilitates an enhanced understanding of the impact of climate-driven environmental changes that is necessary for developing appropriate management strategies for the conservation and sustainable utilization of Arctic marine systems.

Aim We conduct the first model‐based assessment of the biogeographical subdivision of Eurasian Arctic seas to (1) delineate spatial distribution and boundaries of macrobenthic communities on a seascape level; (2) assess the significance of environmental drivers of macrobenthic community structures; (3) compare our modelling results to historical biogeographical classifications; and (4) couple the model to climate scenarios of environmental changes to project potential shifts in the distribution and composition of macrobenthic communities by 2100. Location Eurasian Arctic seas, in particular Barents, Kara and Laptev Seas. Taxon 169 species of macrobenthic fauna; most common taxa are Polychaeta (85 species), Malacostraca (30 species), Bivalvia (26 species) and Gastropoda (10 species). Methods We employed the Region of Common Profile (RCP) approach to assess the bioregionalization patterns of Eurasian Arctic seafloor communities. The RCP approach allows the identification of seascape‐scale distribution patterns by simultaneously considering biotic and environmental data within one modelling step. Results Four RCPs were identified within the Eurasian Arctic. The results showed that water depth, sea‐ice cover, bottom‐water temperature and salinity, proportion of fine sediments, particulate organic carbon (POC) and depth of the euphotic zone were among the most important driving variables of macrobenthos communities. The projections, driven by the climate‐change scenarios, suggested a general north‐eastward shift of the RCPs over the 21st century, mainly correlated with retreating sea‐ice and increasing sea‐bottom temperature. Main conclusions The identified RCPs largely match the previously reported large‐scale distribution patterns of macrobenthic communities in Eurasian Arctic seas. The spatio‐temporal dynamics of RCPs are in agreement with local long‐term observation data on macrobenthic resilience/vulnerability in the studied region. The representation of the ecoregions and biotas in a probabilistic form, together with quantitative assessment of potential climate‐driven changes, will help to adequately consider macrobenthic biodiversity dynamics in the development of science‐based conservation measures.